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ZEUS Collaboration

Publications and source records attributed to ZEUS Collaboration.

At least 145 records · Page 8Linked to original sources
arXiv2002

Measurement of subjet multiplicities in neutral current deep inelastic scattering at HERA and determination of $α_s$

The subjet multiplicity has been measured in neutral current e+p interactions at Q**2 > 125 GeV**2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-1. Jets were identified in the laboratory frame using the longitudinally invariant K_T cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale y_cut. Measurements of the mean subjet multiplicity, < n_sbj >, for jets with transverse energies E_T,jet >15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of alpha_s(M_Z), determined from < n_sbj > at y_cut=10**-2 for jets with 25 < E_T,jet < 71 GeV, is alpha_s (M_Z) = 0.1187 +/- 0.0017 (stat.) +0.0024 / -0.0009 (syst.) +0.0093 / -0.0076 (th.).

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arXiv2002

Searches for excited fermions in ep collisions at HERA

Searches in ep collisions for heavy excited fermions have been performed with the ZEUS detector at HERA. Excited states of electrons and quarks have been searched for in e^+p collisions at a centre-of-mass energy of 300 GeV using an integrated luminosity of 47.7 pb^-1. Excited electrons have been sought via the decays e*->egamma, e*->eZ and e*->nuW. Excited quarks have been sought via the decays q*->qgamma and q*->qW. A search for excited neutrinos decaying via nu*->nugamma, nu*->nuZ and nu*->eW is presented using e^-p collisions at 318 GeV centre-of-mass energy, corresponding to an integrated luminosity of 16.7 pb^-1. No evidence for any excited fermion is found, and limits on the characteristic couplings are derived for masses below 250 GeV.

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arXiv2002

A ZEUS next-to-leading-order QCD analysis of data on deep inelastic scattering

Next-to-leading order QCD analyses of the ZEUS data on deep inelastic scattering together with fixed-target data have been perfomed, from which the gluon and the quark densities of the proton and the value of the strong coupling constant, alpha_s(M_Z), were extracted. The study includes a full treatment of the experimental systematic uncertainties including point-to-point correlations. The resulting uncertainties in the parton density functions are presented. A combined fit for alpha_s(M_Z) and the gluon and qurak densities yields a value of alpha_s(M_Z) in agreement with the world average. The parton density functions derived from ZEUS data alone indicate the importance of HERA data in determining sea quark and gluon distributions at low x. The limits of applicability of the theoretical formalism have been explored by comparing the fit predictions to ZEUS data at very low Q^2.

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arXiv2002

Measurement of high-Q2 charged current cross sections in e-p deep inelastic scattering at HERA

Cross sections for e-p charged current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV with an integrated luminosity of 16.4 pb-1 using the ZEUS detector at HERA. Differential cross-sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2>200 GeV2. In addition, d2σ/dxdQ2 was measured in the kinematic range 280 GeV2 < Q2 < 30000 GeV2 and 0.015 < x < 0.42. The predictions of the Standard Model agree well with the measured cross sections. The mass of the W boson, determined from a fit to dσ/dQ2, is MW=80.3 \pm 2.1 (stat.) \pm 1.2 (syst.) \pm 1.0 (PDF) GeV.

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arXiv2002

Measurement of High-Q^2 Charged-Current e^+p Deep Inelastic Scattering Cross Sections at HERA

The e^+p charged-current deep inelastic scattering cross sections, $dσ/dQ^2$ for Q^2 between 200 and 60000 GeV^2, and $dσ/dx$ and $dσ/dy$ for Q^2 > 200 GeV^2, have been measured with the ZEUS detector at HERA. A data sample of 47.7 pb^-1, collected at a center-of-mass energy of 300 GeV, has been used. The cross section $dσ/dQ^2$ falls by a factor of about 50000 as Q^2 increases from 280 to 30000 GeV^2. The double differential cross section $d^2σ/dxdQ^2$ has also been measured. A comparison between the data and Standard Model (SM) predictions shows that contributions from antiquarks ($\bar{u}$ and $\bar{c}$) and quarks (d and s) are both required by the data. The predictions of the SM give a good description of the full body of the data presented here. A comparison of the charged-current cross section $dσ/dQ^2$ with the recent ZEUS results for neutral-current scattering shows that the weak and electromagnetic forces have similar strengths for Q^2 above $M^2_W, M^2_Z$. A fit to the data for $dσ/dQ^2$ with the Fermi constant $G_F$ and $M_W$ as free parameters yields $G_F = (1.171 \pm 0.034 (stat.) ^{+0.026}_{-0.032} (syst.) ^{+0.016}_{-0.015} (PDF)) \times 10^{-5} GeV^{-2}$ and $M_W = 80.8 ^{+4.9}_{-4.5} (stat.) ^{+5.0}_{-4.3} (syst.) ^{+1.4}_{-1.3} (PDF) GeV$. Results for $M_W$, where the propagator effect alone or the SM constraint between $G_F$ and $M_W$ have been considered, are also presented.

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arXiv2002

Search for RP-violating SUSY and excited fermions at HERA

Recent results from searches for physics beyond the Standard Model (SM) in $e^\pm$-proton collisions at a center of mass energy of 300 -- 318 GeV at HERA are presented. Searches for excitations of fermions and for the production of squarks in R-parity-violating Supersymmetry (SUSY) are reviewed.

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arXiv2002

Measurement of diffractive production of D*+-(2010) mesons in deep inelastic scattering at HERA

Diffractive production of D*+-(2010) mesons in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 44.3 pb-1. Diffractive charm production is identified by the presence of a large rapidity gap in the final state of events in which a D*+-(2010) meson is reconstructed in the decay channel D*+ -> (D0 -> K-pi+) pi+ (+ charge conjugate). Differential cross sections when compared with theoretical predictions indicate the importance of gluons in such diffractive interactions.

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arXiv2002

Inclusive jet cross sections in the Breit frame in neutral current deep inelastic scattering at HERA and determination of alpha_s

Inclusive jet differential cross sections have been measured in neutral current deep inelastic e+p scattering for boson virtualities Q**2>125 GeV**2. The data were taken using the ZEUS detector at HERA and correspond to an integrated luminosity of 38.6 pb-1. Jets were identified in the Breit frame using the longitudinally invariant K_T cluster algorithm. Measurements of differential inclusive jet cross sections are presented as functions of jet transverse energy (E_T,jet), jet pseudorapidity and Q**2, for jets with E_T,jet>8 GeV. Next-to-leading-order QCD calculations agree well with the measurements both at high Q**2 and high E_T,jet. The value of alpha_s(M_Z), determined from an analysis of dsigma/dQ**2 for Q**2>500 GeV**2, is alpha_s(M_Z) = 0.1212 +/- 0.0017 (stat.) +0.0023 / -0.0031 (syst.) +0.0028 / -0.0027 (th.).

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arXiv2002

Measurement of the Q2 and energy dependence of diffractive interactions at HERA

Diffractive dissociation of virtual photons, gamma* p-->Xp, has been studied in ep interactions with the ZEUS detector at HERA using an integrated luminosity of approx. 10 pb^-1. The data cover photon virtualities 0.17 < Q^2< 0.70 GeV^2 and 3 < Q^2< 80 GeV^2 with 3<M_X<38 GeV, where M_X is the mass of the hadronic final state.

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arXiv2002

Leading neutron production in $e^+p$ collisions at HERA

The production of neutrons carrying at least 20% of the proton beam energy ($\xl > 0.2$) in $e^+p$ collisions has been studied with the ZEUS detector at HERA for a wide range of $Q^2$, the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, $e p\to e' X n$, is measured relative to the inclusive cross section, $e p\to e' X$, thereby reducing the systematic uncertainties. For $\xl >$ 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the $γp$ system. For $0.64 < \xl < 0.82$, the rate of neutrons is almost independent of the Bjorken scaling variable $x$ and $Q^2$. However, at lower and higher $\xl$ values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, ${{F}^{\rm\tiny LN(3)}_2}(x,Q^2,\xl)$, rises at low values of $x$ in a way similar to that of the inclusive \ff of the proton. The total $γπ$ cross section and the structure function of the pion, $F^π_2(x_π,Q^2)$ where $x_π= x/(1-\xl)$, have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed $Q^2$, $F^π_2$ has approximately the same $x$ dependence as $F_2$ of the proton.

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arXiv2002

Measurement of proton-dissociative diffractive photoproduction of vector mesons at large momentum transfer at HERA

Diffractive photoproduction of vector mesons, gamma p --> V Y, where Y is a proton-dissociative system, has been measured in ep interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb^-1. The differential cross section, ds/dt, is presented for -t<12 GeV^2, where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, ds/dt ~ (-t)^{-n}. The slope of the Pomeron trajectory, measured from the W dependence of the rho^0 and phi cross sections in bins of t, is consistent with zero. The ratios ds_(gamma p --> phi Y)/dt to ds_(gamma p --> rho^0 Y)/dt and ds_(gamma p --> J/psi Y)/dt to ds_(gamma p --> rho^0 Y)/dt increase with increasing -t. Decay-angle analyses for rho^0, phi and J/psi mesons have been carried out. For the rho^0 and phi mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models.

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arXiv2002

Dijet photoproduction at HERA and the structure of the photon

The dijet cross section in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb$^{-1}$. The events were required to have a virtuality of the incoming photon, $Q^2$, of less than 1 GeV$^2$ and a photon-proton centre-of-mass energy in the range $134 < W_{γp} < 277$ GeV. Each event contains at least two jets satisfying transverse-energy requirements of $E_{T}^{\rm jet1}>14$ GeV and $E_{T}^{\rm jet2}>11$ GeV and pseudorapidity requirements of $-1<η^{\rm jet1,2}<2.4$. The measurements are compared to next-to-leading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested.

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arXiv2002

Measurement of the photon-proton total cross section at a center-of-mass energy of 209 GeV at HERA

The photon-proton total cross section has been measured in the process e+ p -> e+ gamma p -> e+ X with the ZEUS detector at HERA. Events were collected with photon virtuality Q^2 < 0.02 GeV^2 and average gamma-p center-of-mass energy W_{gamma p} = 209 GeV in a dedicated run, designed to control systematic effects, with an integrated luminosity of 49 nb^{-1}. The measured total cross section is sigma_{tot}^{gamma p} = 174 +- 1 (stat.) +- 13 (syst.) microbarns. The energy dependence of the cross section is compatible with parameterizations of high-energy p-p and p-pbar data.

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arXiv2002

Search for lepton-flavor violation in e+p collisions at HERA

A search has been made for lepton-flavor-violating interactions, e+ p -> mu/tau X, at a center-of-mass energy of 300 GeV with an integrated luminosity of 47.7 pb^-1 using the ZEUS detector at HERA. No evidence was found for lepton-flavor violation and constraints were derived on leptoquarks that could mediate such interactions. Some of the constraints are also applicable to lepton-flavor-violating processes mediated by squarks in R-parity-violating supersymmetric models. In some cases involving heavy quarks and especially for the tau channel, the ZEUS constraints are the most stringent published to date.

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arXiv2001

High-mass dijet cross sections in photoproduction at HERA

Dijet differential cross sections for the reaction e+p -> e+ + jet + jet + X in the photoproduction regime have been measured with the ZEUS detector at HERA using an integrated luminosity of 42.7 pb**{-1}. The cross sections are given for photon-proton centre-of-mass energies in the range 134< W <277 GeV. The differential cross sections as a function of the dijet mass, Mjj, and of the dijet angular variables have been measured for 47 < Mjj < 160 GeV and compared to next-to-leading-order QCD calculations. The dijet events in the region 75< Mjj < 100 GeV have been used to derive a 95% C.L. upper limit on the cross section for Z0 photoproduction of sigma(e+ p -> e+ Z0 X} < 5.9 pb. Upper limits on the photoproduction of new heavy resonances decaying into two jets are also presented for masses in the range between 60 GeV and 155 GeV.

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arXiv2001

Experimental Results on the Diffractive Production of Light Vector Mesons at HERA

We discuss results on the diffractive production of the vector mesons rho, phi and omega reported by the H1 and ZEUS collaborations at HERA. Unique to such studies is the experimental accessibility to the polarization of the vector mesons and hence to the spin-density matrix elements arising in vacuum-exchange processes. We emphasize the relation between the observed dependence on momentum transfer and the polarization state of the vector meson. The diffractive nature of the production mechanism is investigated via extraction of the Pomeron trajectory at high Q^2. Flavor symmetry is observed in the phi/rho ratios in the same region of momentum transfer where the power-law scaling becomes similar. The multivariable helicity analyses impose stringent constraints on models for the vacuum-exchange production mechanism. Semi-exclusive photoproduction of transverse rho and phi mesons at momentum transfers far exceeding their mass scale exhibit a hard scaling behavior which appears to violate the QCD helicity selection rules in a two-gluon exchange model.

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arXiv2001

Dijet production in neutral current deep inelastic scattering at HERA

Dijet cross sections in neutral current deep inelastic ep scattering have been measured in the range $10 < \Q2 < 10^4$ GeV$^2$ with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb$^{-1}$. The cross sections, measured in the Breit frame using the $\kt$ jet algorithm, are compared with next-to-leading-order perturbative QCD calculations using proton parton distribution functions. The uncertainties of the QCD calculations have been studied. The predictions are in reasonable agreement with the measured cross sections over the entire kinematic range.

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